The present invention relates to a method for separating a sample and a method for preparing the separated sample for analysis, in cases where analysis is desired; and particularly relates to a method for separating a minute sample region from a substrate such as a semiconductor wafer.
This application describes embodiments in which a sample is cut out of a semiconductor wafer or other object by use of a focused ion beam (xe2x80x9cFIBxe2x80x9d) and analyzed, if desired, through a transmission electron microscope (xe2x80x9cTEMxe2x80x9d), or by other means.
In a conventional method for TEM sample preparation, a chip, or ribbon, having a length of several mm and a width of 100-500 xcexcm is cut out from a semiconductor integrated circuit wafer by use of a diamond wafering saw. The chip is mounted on a standard TEM grid. Then the chip is formed into a thin sample (typically 50 xcexcm) by the FIB. The thin film sample is irradiated with an electron beam and observed by use of the TEM.
In conventional TEM observations a sample is thinned by polishing, while being observed. With this method, it is difficult to set the place of observation and direction of the sample desirably and precisely. It is necessary to carry out a step in which a region having a length of several mm and a width of 100-500 xcexcm, and including a portion to be analyzed, is mechanically separated from the chip of an integrated circuit or semiconductor wafer. When a wafer is a sample substrate, it is necessary to divide the wafer for observation. It is difficult to process a sample to have a thickness less than 100 xcexcm through mechanical processing by means of a diamond wafering saw or the like, because it is difficult to maintain accuracy and avoid damage. Portions which could not be cut to be thin enough through mechanical processing, would then have to be thinned with the FIB, which adds to processing time.
In another conventional method, the sample is cleaved first. The plane of cleavage is then observed by a scanning-electron microscope. This method however, makes it difficult to specify a desired portion precisely, and makes it difficult to observe the section cleaved. All of the foregoing conventional techniques have disadvantages because it is difficult to make a section even substantially flat and parallel to the sample surface for observation, and it is impossible to observe, for example, a horizontal section of a contact hole.
Another conventional method uses the FIB to cut a sample from a wafer by cutting the sample from at least two different angles after a probe has been attached to the sample. The probe then removes the sample for analysis. This method has the disadvantage that the wafer must be moved to a second angle for the second cut to be effected. These techniques typically rely on a break in electrical conductivity or trial and error to tell when the sample is isolated.
There is a need for a method of sample separation which can produce precise samples, but which does not need multiple cuts. There is a particular need for a sample-separation method which will allow the operator to see the operation in process and to clearly see when the sample separates, without the need for electrical contact tests, or the like.
The invention is a method for sample separation and lift-out within a FIB instrument. The preferred embodiment comprises the steps of, first, cleaving a wafer, having an area of interest, or target, to be removed, so as to place the target near the edge of the cleaved wafer.
Then, the tip of a micromanipulator probe is fixed to the wafer by ion-beam metal deposition. The FIB ion-beam is positioned at an angle of approximately 50 degrees to the plane of the wafer, and the ion beam cuts a U-shaped path on the surface of the wafer, so that the path surrounds the target and the fixed probe, and completely released the sample from the wafer. Next, the stage of the FIB is lowered slightly, so that the wafer is lowered beneath the released sample, now attached to the probe.
The probe with the sample fixed to it is moved to a TEM grid. The TEM grid is preferably cut across its plane so as to provide a V-shape open above. The opening allows easy access to the V-shape for placement of the sample and further operations with the ion beam. The sample is then fixed to the TEM grid by ion-beam metal deposition, and the probe is then cut by the ion-beam and detached from the fixed sample. The sample may now be thinned by the ion beam for TEM inspection, or inspected by other means. The reader should note that with the method just described, only one ion-beam cut at one angle is necessary to release the sample from the wafer and allow it to be lifted out.